1. Field of the Invention
The present invention relates in general to an integrated circuit fabrication. In particular, the present invention relates to a dual salicidation process that can form a silicide gate conductor having a greater thickness than a silicide structure on a source/drain region.
2. Description of the Related Art
In integrated circuit fabrication, the gate conductor is commonly used as a channel region mask during the formation of the source and drain junctions. One of the disadvantages of using polysilicon as the gate conductor material, however, is that it has a significantly higher resistivity than metals, such as aluminum. The propagation delay of an integrated circuit employing a polysilicon gate conductor thus may be longer than desired. Consequently, the operational frequency that can be achieved by a circuit employing a polysilicon gate conductor is somewhat limited.
To reduce the contact resistance at the contact/junction and contact/gate conductor interfaces, self-aligned low resistivity structures are commonly placed between the ohmic contacts and the junctions/gate conductors. The presence of these so-called self-aligned silicides (i.e., salicides) upon the junctions and gate conductors ensures that contact is made to the entire junction and gate areas. Further, forming salicide upon a polysilicon gate conductor helps lower the sheet resistance of the gate conductor. Salicide formed upon polysilicon is generally referred to as polycide.
Transistor device dimensions have been continuously reduced to accommodate the high demand for faster, more complex integrated circuits. As such, the source and drain junction depths have been reduced. Unfortunately, a salicide may completely consume a relatively shallow junction and penetrate into the substrate underneath the junction, a phenomenon known as xe2x80x9cjunction spikingxe2x80x9d. Junction spiking may undesirably cause the junction to exhibit large current leakage or become electrically shorted. Therefore, in order to prevent excessive consumption of shallow junctions during contact formation, the junction salicide can only be of limited thickness. Since the gate and junction salicides are formed at the same time, the gate salicide also has a limited thickness. However, it is desirable to form a relatively thick layer of salicide upon a gate conductor to lower the sheet resistance of the gate conductor. Accordingly, it would be of benefit to develop a salicidation process in which the junction salicides and the gate salicides have dissimilar thicknesses. That is, the salicidation process must no longer require concurrent formation of the junction salicides and the gate salicides.
Please refer to FIGS. 1A to 1I, which depict a dual salicidation process according to the prior art. As shown in FIG. 1A, a semiconductor substrate 10 comprises shallow trench isolation structures 12 arranged a spaced distance apart for isolating active areas, a gate dielectric 14 formed on the substrate 10, and a polysilicon gate conductor 16 patterned on the gate dielectric 14 by using well-known lithography and etch techniques. The gate dielectric 14 is made of a material having a K value greater than approximately 4. The gate conductor 16 is made by polysilicon.
Next, as shown in FIG. 1B, source-side/drain-side LDD areas 18 are formed by self-aligning an LDD implant to the opposed sidewall surfaces of gate conductor 16. Next, as shown in FIG. 1C, a dielectric material is deposited on the substrate 10 and then an anisotropical etching process is performed on the dielectric material. As a result, the dielectric material is only retained laterally adjacent the sidewalls surfaces of the gate conductor 16 in the form of sidewall spacers 22.
As shown in FIG. 1D, a S/D implant that is self-aligned to the outer lateral surfaces of the sidewall spacers 22 is then performed at a higher dose and energy than the LDD implant. In this manner, source and drain regions 24 are formed within substrate 10 a spaced distance from gate conductor 16. As such, LDD areas 18 and source and drain regions 24 form graded junctions which increase in concentration in a lateral direction away from gate conductor 16.
Thereafter, as shown in FIG. 1E, a first layer of refractory metal 26 is deposited across exposed surfaces of gate dielectric 14, sidewall spacers 22, and gate conductor 16. The first layer of refractory metal 26 may be made of cobalt and titanium. The first layer of refractory metal 26 may be subjected to radiation 28 to cause the metal atoms of the first layer of refractory metal 26 to undergo cross-diffusion and reaction with silicon atoms within polysilicon gate conductor 16. As a result, a majority of polysilicon gate conductor 24 may be converted into a silicide gate conductor 30, as shown in FIG. 1F. The excess refractory metal not consumed during this salicidation process is removed using a selective etch technique. The resulting silicide gate conductor 30 comprises TiSi2 if Ti is used as the refractory metal and CoSi2 if Co is used as the refractory metal.
Turning to FIG. 1G, the gate dielectric 14 may then be removed from source and drain regions 24. Subsequent to exposing the source and drain regions 24, a second layer of refractory metal 32, e.g., titanium or cobalt, may then be deposited across the semiconductor topography, as shown in FIG. 1H. The second layer of refractory metal 32 is substantially thinner than the first layer of refractory metal 26. The topography may then be exposed to radiation 34 to heat the second layer of refractory metal 32. As a result of being annealed, metal atoms within the second layer of refractory metal 32 may react within underlying Si atoms of substrate 10. In this manner, silicide structures 36 comprising, e.g., TiSi2 or CoSi2 are formed upon the source and drain regions 36, as shown in FIG. 1I. Any non-reacted refractory metal may be selectively etched away.
An object of the present invention is to provide a dual salicidation process to form a relatively thick layer of the silicide gate conductor to lower the sheet resistance of the gate conductor.
The dual salicidation process of the present invention includes the steps of: covering a sacrificial layer on the top of a polysilicon gate conductor; performing a thermal oxidization process to form a poly-oxide spacer on the sidewall of the polysilicon gate conductor; forming source/drain regions within the substrate at the outer lateral surfaces of the poly-oxide spacer ; removing the sacrificial layer; performing a first salicidation process to convert the polysilicon gate conductor to a silicide gate conductor; performing a second salicidation process to form silicide structures upon the source/drain regions.
It is an advantage of the present invention that the two-step salicidation process ensures that excessive consumption of source/drain regions does not occur during the formation of silicide gate conductor. Also, it is desirable to form a relatively thick layer of the silicide gate conductor to lower the sheet resistance of the gate conductor. Accordingly, the silicide structures and the silicide gate conductor have dissimilar thicknesses. Besides, the poly-oxide spacer is self-aligned and formed by the thermal oxidization process and thereby the steps of depositing dielectrics and anisotropical etching process can be omitted. This can simplify the dual salicidation process and is applied to fabricating a smaller-scale gate conductor.